Honestly, it seems like there is a major refurb of some sort every month. The park is really busy year round so I am not sure summer is the busy season as it once was. It seems to alway be busy season.
Haunted Mansion always goes down for about 2 weeks in August to put the holiday overlay on. This year it is going down for a long refurb and no one knows when it will be back up. Some say not until the ride reopens for the holiday version in mid August, but I believe my CM friend who works at HM who thinks late or may or early June for it to come up. But then it will still need to go back down in August to prepare for HMH.
Also, the second half of August is a bit quieter as many schools in the area go back around the second to third week of August.